I used some feeder guppies from my turtles tank to cycle my last 2 tanks. They took it fine. If one did die, I would have just feed it to my turtle anyways, but they made it through. I think part of it was the fact that they were shrimp tanks and I used Fluval Shrimp Stratrum which lowered my 7.6pH tap water to 6.6pH and my understanding from reading, is that free hydrogen atoms in a low pH water will bind with ammonia and form ammonium, which is pretty much harmless to fish, but still will feed the biological bacteria in the filter and thus allow the cycle to complete.